OBJECTIVES: 1. Complete studies of the morphology of skin allograft rejection in man. 2. Extend these studies to the mouse, where genetically defined strains are available for critical studies of both acute and chronic rejection. 3. Nature of the inflammatory cell infiltrate and other fine structural alterations in primary cutaneous halo nevi. This entity may mimic malignant melanoma except that the lesions are characterized by spontaneous regression. Therefore, it provides an elegant model for study of the mechanisms by which humans reject tumors akin to melanoma.